There has been an ongoing tendency in the Photovoltaic Industry, to continually reduce silicon substrate thicknesses. Improved techniques, such as the introduction of wire sawing has had a big impact in terms of producing thinner substrates with smaller kerf losses. However, back contact formation presents some problems with thinner substrates with the effective rear surface recombination velocity (RSRV) becoming of relative greater importance. Few, if any, current commercial techniques have good enough rear surface passivation (low enough RSRV) to prevent performance loss with thinner devices.
One alternative approach which has been considered from time to time over the past decade, is to use simple abrasion of the rear surface to expose the pyramid peaks of the rear surface texturing, therefore potentially facilitating contact to the underlying p-type material. Some of these ideas were published in the mid 1980's (Wenham, PhD thesis, The University of New South Wales, 1986, P171), although a suitable technology/processing sequence for the implementation of the ideas, has not previously been identified. One of the main reasons for this is linked to the preference for using a rear n-type layer for both simplicity (since it can be simultaneously formed with the front n-type layer) and performance enhancement (via a lower effective RSRV). However, problems occur when trying to penetrate through the n-type layer to the p-type underlying material via a boron diffusion. Furthermore, the high temperatures associated with boron diffusion can often damage or degrade commercial substrates.
Silicon has a very high mobility in aluminium, even at temperatures well below the aluminium/silicon eutectic temperature of 577.degree. C. It has been known for some time that amorphous silicon can penetrate through an aluminium layer and epitaxially grow or deposit onto a crystalline silicon surface on the opposite side of the aluminium, at temperatures well below the eutectic temperature (Majni and Ottavian; Applied Physics letters, Vol 51., No. 2, Jul. 15, 1977 pp 125-126). This effect is evident whether the crystalline silicon is mono-crystalline or poly-crystalline. The same end results are achieved in the event that the positions of the aluminium and amorphous silicon layers are reversed with the silicon again initially penetrating into the aluminium when heated prior to epitaxially growing on the newly exposed crystalline silicon surface. Similar results have also been observed for germanium.
Based on this rather extraordinary mechanism for solid phase epitaxial growth at low temperature, a new contacting scheme for crystalline devices is proposed whereby the above growth approach is used to enable p-type contacts to be made to the substrate.
This approach is particularly useful for forming p-type contacts through a rear n-type floating junction, the benefits of which are often destroyed by other low cost contacting methods, which usually short out the floating junction. The presently proposed method enables p-type contacts to be made to the substrate while simultaneously forming a rectifying junction to the rear n-type layer, thereby leaving the n-type layer isolated. The rear contact area can be restricted to about 1% of the rear surface with the remainder being well passivated by the rear floating junction in conjunction with a high quality thermal oxide or independently deposited dielectric. The low temperatures involved in this process are particularly attractive for wafers grown by the Czochralski technique with the corresponding well passivated rear surface being particularly well suited for thin Czochralski wafers in commercial production.